For homo- or copolymerization of olefins a number of processes using a catalyst system comprising a solid catalyst component essentially containing magnesium, titanium, an electron donor compound, and a halogen, an organoaluminum compound, and an organosilicon compound have been proposed.
For example, JP-A-57-63310 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-57-63311 propose a process for polymerizing an olefin having 3 or more carbon atoms using a catalyst system comprising a solid catalyst component containing a magnesium compound, a titanium compound and an electron donor, an organoaluminum compound, and an organosilicon compound having an Si-O-C bond. However, these processes are not always satisfactory for obtaining highly stereoregular polymers in high yield, and further improvement has been demanded.
On the other hand, JP-A-63-3010 discloses a catalyst system for the polymerization of olefins and a process for polymerizing olefins using the same, the catalyst system comprising (a) a solid catalyst component prepared by bringing a dialkoxymagnesium, an diester of aromatic dicarboxylic acid, an aromatic hydrocarbon, and a titanium halide into contact and subjecting the resulting product in a powdered state to a heat treatment, (b) an organoaluminum compound, and (c) an organosilicon compound.
JP-A-1-315406 discloses a catalyst system for olefin polymerization and a process for polymerizing an olefin using the same, the catalyst system comprising (a) a solid catalyst component prepared by bringing titanium tetrachloride into contact with a suspension of diethoxymagnesium in an alkylbenzene, adding phthalic acid dichloride thereto to react to obtain a solid product, and further contacting the resulting solid product with titanium tetrachloride in the presence of an alkylbenzene, (b) an organoaluminum compound, and (c) an organosilicon compound.
JP-A-2-84404 proposes a catalyst system for the polymerization of olefins and a process for homo- or copolymerizing an olefin(s) using the same, the catalyst system comprising (a) a solid titanium catalyst component essentially containing magnesium, titanium and a halogen which is prepared by bringing a magnesium compound and a titanium compound into contact, (b) an organoaluminum compound catalyst component, and (c) an organosilicon compound catalyst component containing a cyclopentyl group or a derivative thereof, a cyclopentenyl group or a derivative thereof, or a cyclopentadienyl group or a derivative thereof.
Each of these known techniques aims at such high catalytic activity that a step of removing residual catalyst components, such as chlorine and titanium, from the resulting polymer (a so-called ashing step) may be omitted and, at the same time, an improvement in yield of a stereoregular polymer or an improvement in durability of the catalytic activity for polymerization, and has achieved excellent results to their purpose.
In recent years, however, it has been pointed out that the olefin polymers obtained by polymerization using these catalyst systems comprising such a highly active catalyst component, an organoaluminum compound and an organosilicon compound have narrower molecular weight distribution as compared with those obtained by using conventional catalyst systems comprising a titanium trichloride type catalyst component in combination with an organoaluminum compound and, if desired, an electron donor compound as a third component. For polyolefins to have narrower molecular weight distribution means poorer moldability, leading to less applicability.
Various manipulations have been suggested to solve this problem. For example, adoption of a multi-stage polymerization system has been proposed for obtaining polyolefins with broader molecular weight distribution. Nevertheless, a multi-stage polymerization system requires repetition of tedious and complicated operation of polymerization and also involves a step for recovery of a chelating agent to be used for polymerization and is not therefore deemed to be favorable from the considerations of labor and cost.
As the latest technique, JP-A-3-7703 discloses a process for polymerizing an olefin in the presence of a catalyst system comprising (a) a solid titanium catalyst component essentially containing magnesium, titanium, a halogen, and an electron donor, (b) an organoaluminum compound, and (c) at least two organosilicon compounds as an electron donor. According to this process, a polyolefin having broad molecular weight distribution can be obtained without involving laborious multi-stage polymerization operation. However, the use of at least two organosilicon compounds as an electron donor for polymerization makes the process still tedious and complicated.